The object of the invention is a device for dispensing liquids. The device according to the invention comprises a liquid tank, a dispensing nozzle, and a liquid conduit with an end connected to the liquid tank for supplying liquid from the liquid tank to the dispensing nozzle. The device is especially suited for the accurate dispensing of small amounts of liquid, in particular dyes, medicine or similar liquid that must be dispensed in very exact, controlled portions. The invention also relates to a printing head and printing system using the inventive dispensing device.
Various types of liquid dispensing devices having a dispensing nozzle are known in the art. U.S. Pat. No. 3,653,598 to Waldrum discloses a vibrating spray apparatus designed for use in agricultural spray systems. This known apparatus comprises a spray tube with an associated vibration transmitting device. The free end of the spray tube is caused by the vibration transmitting device to move in an orbital path. Liquid is discharged from the spray tube during the vibration, under the effect of the pressure in a remote liquid tank. This known apparatus is suitable for providing a very uniform distribution of the liquid during the spraying. The device is not suitable for dispensing small amounts of liquid in a well-controlled manner.
Printing technology is a special area where various liquid dispensing techniques are used. Printed images on paper or other substrates is in constant demand, which is supported strongly by IT and printed products are used daily in practically all areas of the economy. The demand is likely to remain or even increase in the future as well. Significant efforts are invested in the development of high-speed and cost-saving printing systems. Research is largest in two different directions. The first is the combination of conventional printing technologies with digital pre-pressing, and the second one is the development of entirely digitised printing systems.
The conventional offset printing system, for instance, is advantageous from the cost-benefit point of view only if high volumes are printed. The cost of pictures printed with modern digital systems are less dependant on volume, once the systems are installed. However, the large initial costs mean that the total production costs per piece are still higher as opposed to the conventional systems. As a further disadvantage, the dyes of the currently used ink-jet and bubble-jet printing technologies are inferior to traditional techniques with respect to water and UV resistance. While desktop colour printers are becoming commonplace, at the same time, there is a long-felt need for printing techniques which would make possible the cost-effective, fast printing of very few, even single items, combined with a capacity to print large-scale, i. e. large sized products. Examples of such products are large posters for advertising purposes.
Liquid dispersing or spraying technology using ultrasound generated by a piezo-electric transducer is known in the art. In simple terms, ultrasound liquid dispensing is based on the following phenomenon: If a mechanical vibration with a high amplitude can be achieved it is capable of dispersing the liquid to drops by overcoming the surface tension. There are two basic types of ultrasound liquid dispersion:
High frequency (approx. 1 MHz or higher) vibration energy radiating from the transducer is concentrated in the liquid in order to achieve the necessary energy density. i. e. pressure, for dispersion.
In the techniques involving a lower frequency, the necessary energy density is achieved range by using different types of solid concentrators, and the liquid is lead to a surface. which is vibrating at a relatively high amplitude.
The presently available ultrasonic liquid dispersers have a number of disadvantages. Their external dimensions are rather large, and therefore their application in the printing industry is limited. Also, because of their large size, the vibrating mass is also large, which results in a long activation time. Besides all these disadvantages, the problem of adjusting the pixel size created by the ejected liquid droplets is not solved. Cleaning of the device, replacement of the parts and a relatively complicated electric system cause problems as well.
U.S. Pat. No. 4,815,661 to Anthony relates to an ultrasonic spraying device with a body and a piezo-electric vibrating core. The vibrations generated by the vibrating core are transmitted to a spray nozzle. The liquid to be sprayed is atomised by the vibration of the spray nozzle. The liquid is sprayed out as the result of the internal pressure within the liquid, the internal pressure being caused by the vibrating core.
U.S. Pat. No. 4,897,673 to Okabayashi et al. teaches a method for connecting a nozzle tube of an ink jet printer with a piezoelectric element. There is disclosed a nozzle tube in operating connection with a piezoelectric element, which latter causes the periodic contraction and expansion of the nozzle tube, and thereby the discharge of liquid (ink) from the nozzle tube. As above, the liquid is discharged under pressure which is created within the nozzle tube.
As mentioned above, these known techniques are not suitable for large-scale, fast printing. Therefore, the principal objectives of the invention are the following:
Achieving a liquid drop size, or a pixel size on paper or on other substrates, which could be varied between wide ranges. Most specifically, it is sought to provide a device which produces a variable tone on the printed pixels.
Creating a dispensing device with a reduced size in at least one dimension, allowing the positioning of the liquid dispensing units closely next to each other. In this manner, continuous parallel printing could be achieved in a full width of a printing substrate. In order to achieve high printing speeds, it was sought to reduce the time of creating a liquid drop.
Further, it was sought to provide a system without the need for an elaborate electric system, combined with a possibility to apply widely different liquid types. Also, it was desired to create a system which can be connected to and controlled by a computer, and solves the problems of cleaning and part replacement.
According to a first aspect of the invention, the above goals are achieved with a device comprising a liquid tank, a dispensing nozzle, and a liquid conduit with an end connected to the liquid tank for supplying liquid from the liquid tank to the dispensing nozzle. According to the invention, the device further comprises vibrator means for inducing a vibrating action of the dispensing nozzle, and by the vibrating action resulting in an acceleration of the nozzle sufficient to cause the detachment of the liquid from the nozzle.
The invention also extends to a liquid dispensing device with essentially the same basic features, but without the liquid tank. This modified inventive device connects to an external liquid tank.
In a preferred embodiment, the liquid conduit is integral with the dispensing nozzle. Advantageously, a free end of the liquid conduit is cut at an angle, and the cut free end functions as the dispensing nozzle. Alternatively, the nozzle could be formed as a free end of the liquid conduit having an decreasing diameter towards the free end, at least in a part of the conduit adjacent to the free end.
In the most preferred embodiment, the vibrator means comprises a piezo-electric transducer, and the liquid conduit is a hollow metal tube. In this case the nozzle is at the vibrating end of the tube. The end may be cut at an angle or its cross-section may be gradually decreasing.
In the device according to the invention, it is foreseen that the transducer is attached to the liquid conduit through a resonator. It is most preferred that the transducer, the resonator and the conduit constitute a resonating unit.
In a specific embodiment, the resonator is a flat steel plate attached directly or indirectly to the liquid conduit adjacent to the free end. Its major advantage over the three dimensional i.e. the spatial resonators is not only the small lateral size but also the much shorter activation time, which allows the pulsed operation of the liquid dispensing apparatus and makes a controlled liquid transport in a short time possible.
The plate-resonator of the invention is fastened to the liquid conducting tube which has a smaller weight than the resonator itself, and therefore the tube vibrates at a higher amplitude. The nozzle is formed at a free end on the liquid conducting tube, and vibrates at the maximum amplitude. However, this system dispenses liquid properly if the right amount of liquid is directed to the active parts i.e. to the nozzles. If this quantity is more or less than the optimal, the capacities of the system remain unexploited. The liquid supply to the nozzle is influenced by the capillary effect and the hydrostatic pressure in the tube. Since the capillary effect is difficult to control, it is suggested that the device should comprise means for varying the hydrostatic pressure of the liquid in the liquid tank and/or in the liquid conduit.
In a most preferred embodiment of the device according to the invention, the transducer is a circular disk-shaped piezoelectric transducer, and the steel plate is substantially drop-shaped with a circular part having a triangular extension integral with the circular part. The transducer is attached parallel to the circular part in a concentric position and the apex of the triangular extension is attached to the liquid conduit.
Facilitating computer control of the device, it may further comprise externally controlled driver means for driving the transducer at predetermined, variable frequencies.
A second aspect of the invention relates to a printing head comprising multiple liquid dispensing means for dispensing dye in controlled amounts in predetermined, controlled locations of a printing medium. According to the invention, the liquid dispensing means comprises liquid dispensing device according to the first aspect of the invention. In a preferred embodiment, the printing head comprises parallel slots for receiving the liquid dispensing devices, and contact springs for fastening the liquid dispensing devices to the wall of the slots.
In order to facilitate variable pixel size and/or variable tone on different substrates, the printing head comprises multiple dispensing devices arranged in a line, and further comprises moving means for translating movement of at least the nozzles of the dispensing devices in a direction parallel to the line. It is also suggested to include adjustment means for an additional translating movement of the nozzles of the dispensing devices in a direction perpendicular to the line, simultaneously or individually for each nozzle. This is especially useful to adjust the pixel size (width) to the pixel resolution (number of pixels per unit length) and/or to the tone (coverage).
The invention also relates to a printing system with a printing medium feeding mechanism and at least one printing head for dispensing dye in controlled amounts in predetermined, controlled locations of the printing medium, with a printing head according to the second aspect of the invention. It is suggested to utilise multiple printing heads, with each printing head dedicated to a predetermined colour.
The present invention is suitable for the continuous or pulsed dispensing of small quantities of liquid. The invention ejects the liquid particles as droplets towards the target medium with a great energy and with a high repetition rate, and the ejected quantities of the liquid can be adjusted accurately. The liquid may be a solvent (e.g. water, acetone, etc.), a dye solution (e.g. ink), emulsion or suspension (e.g. pigmented ink).
Due to the above-mentioned features, the apparatus is best suitable for printing purposes, and the application of the inventive device in printing systems puts a novel printing process into practice. However, pharmaceutical and medical purposes are also considered as areas of application, as well as any other areas where relatively small quantities of liquid have to be dispensed with great accuracy and without contamination.